Probing the topology of a bacterial membrane protein by genetic insertion of a foreign epitope; expression at the cell surface.

The LamB protein is a trimeric integral outer membrane protein from Escherichia coli K12 which functions as a pore for maltodextrins and a receptor for bacteriophages. When inserted into two selected sites of LamB, a foreign antigen, the C3 epitope from poliovirus, was exposed at the cell surface with its normal antigenic properties. Since these genetic insertions did not affect in any essential way the routing, activity and folding of the LamB protein, we conclude that the two corresponding LamB sites are at the cell surface as predicted by our recent model. We discuss the implications of our results for the study of protein topology with a single epitope and the direct cloning and cell surface expression of epitopes of interest as well as the development of live vaccines or diagnostic tests.